Improve tag creation tracking note

[saethlin]

I've been looking at a number of Miri errors recently where tracking the relevant tag results in just two messages like the following:

test join ... note: tracking was triggered
  --> /home/ben/futures-rs/futures-task/src/waker_ref.rs:58:16
   |
58 |     let ptr = (&**wake as *const W) as *const ();
   |                ^^^^^^^ created tag 199076
   |

   --> /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/non_null.rs:327:18
    |
327 |         unsafe { &*self.as_ptr() }
    |                  ^^^^^^^^^^^^^^^ trying to reborrow for SharedReadWrite at alloc74291, but parent tag <199076> does not have an appropriate item in the borrow stack
    |

I suspect that in this case the problem is that the created borrow is SRO for an SRW use, or an SRW tag for a Unique use. It would be good if Miri explained a bit more about the tag.

I'd be submitting the PR to do this, but the diagnostic code is a bit displaced from the information about the tag, so improving this requires a bit more finesse with the code than I have patience for today.

[RalfJung]

I suspect that in this case the problem is that the created borrow is SRO for an SRW use, or an SRW tag for a Unique use.

Sounds plausible. It could also be the case that this pointer is used outside of the range of memory that it is valid for (i.e., it never was valid for that location to begin with, so no "invalidation" diagnostic is printed).

The latter case would be hard to detect, but showing a specific error when a tag exists but with the wrong permission would be a good idea indeed.

[saethlin]

The PR mentioned above takes care of one aspect of this.
I think it manages a pretty good balance of resource usage and helpfulness, but there is a lot more information we could provide when tag-tracking is turned on. For example, it would be very cool to see the tag tracking notes explain when a retag with a shared reference produces SRW over one region and SRO over another. And it would be cool to see 2-phase borrows called out.